A random access procedure is a basic function in LTE (long term evolution system). Only after a user equipment is synchronized with an uplink of a system through the random access procedure can the user equipment be scheduled by the system to perform uplink transmission (for example, transmission of uplink data or transmission of uplink control information). Random access in the LTE is classified into two forms, that is, contention-based random access (that is, a network side does not assign a dedicated random access sequence to the user equipment) and non-contention random access.
As shown in FIG. 1, a contention-based random access procedure includes the following four steps.
First step (MSG1): A user equipment sends random access sequences (random access preamble) selected by the user equipment to a network side over a PRACH (physical random access channel) selected by the user equipment. The random access sequences are divided by the network side into a group A and a group B, and according to the size of a message required to be sent and the loss of a current downlink path of the user equipment, the user equipment decides whether to select and send a random access sequence in the group A or select and send a random access sequence in the group B.
Second step (MSG2): The network side sends a random access response to the user equipment. The random access response includes an index number of a detected preamble sequence, time adjustment information used for uplink synchronization, an initial uplink resource assignment (used for sending a subsequent MSG3) and a temporary C-RNTI (Cell-RNTI, serving cell-radio network temporary identifier, where it is decided whether the temporary C-RNTI is to be converted into a permanent C-RNTI in an MSG4). The user equipment needs to receive the corresponding MSG2 (the random access response) through interception on a PDCCH (physical downlink control channel) scrambled by an RA-RNTI (random access-radio network temporary identifier).
Though the user equipment intercepts the MSG2, because the MSG1 sent by the user equipment is randomly selected from public resources, it is possible that different user equipments send the same MSG1 (random access sequence) over a PRACH resource of the same time and frequency, so that the user equipment needs to solve this random access contention through subsequent MSG3 and MSG4 (a contention resolution message).
Third step (MSG3): The user equipment sends the MSG3 to the network side. The content of the MSG3 is not limited. The MSG3 sometimes may carry an RRC (radio resource control protocol) connection request and sometimes may carry some control information or service data packets, and sometimes, to distinguish different user equipments, the MSG3 may also carry a specific ID of the user equipment. The MSG3 is a message transmitted over a PUSCH (physical uplink shared channel).
Fourth step (MSG4): The network side sends, to the user equipment, a contention resolution message (Contention Resolution). If a user equipment ID carried in the contention resolution message received by the user equipment matches with the specific ID of the user equipment sent by the user equipment in the MSG3, or a carried contention detection identifier matches with the RRC sent by the user equipment in the MSG3, the random access succeeds.
Similar to the contention-based random access procedure, as shown in FIG. 2, a non-contention random access procedure includes the following. First, a network side assigns, to a user equipment, a dedicated random access sequence, and the network side sends, to the user equipment, random access sequence assignment (Random Access Preamble assignment), that is, an MSG0. Next, the user equipment sends, to the network side over a selected PRACH, the dedicated random access sequence, that is, an MSG1, assigned to the user equipment. Finally, the network side sends a random access response, that is, an MSG2, to the user equipment. If the user equipment receives the random access response, the random access succeeds.
After the random access succeeds, the user equipment performs uplink data transmission. When the total power demanded for the uplink data transmission is greater than a maximum power transmission capability of the user equipment, that is, when power is limited, transmission power needs to be scaled. Priorities of power transmission of uplink channels when the power is limited are that a priority of a PUSCH transmission over which does not include UCI (uplink control information) is lower than a priority of a PUSCH transmission over which includes the UCI, and the priority of the PUSCH transmission over which includes the UCI is lower than a priority of a PUCCH (physical uplink control channel).
The current random access procedure is applicable to a situation in which when multiple uplink carriers in carrier aggregation pass through the same path, all the carriers share the same timing advance. The random access procedure is only initiated in a primary serving cell, and in the random access procedure, a situation of other uplink transmission does not exist.
However, when the uplink carriers in the carrier aggregation pass through different paths, all the carriers no longer share the same timing advance, the random access procedure needs to be initiated in an auxiliary serving cell, and meanwhile, other uplink transmission may exist on another synchronized carrier (for example, uplink data transmission is performed in the primary serving cell while a random access procedure is performed in an auxiliary serving cell to obtain the timing advance used in uplink synchronization). At this time, the contention between the uplink data transmission and the random access procedure may occur in the current random access procedure.